Radio communication system

ABSTRACT

In a radio communication system having a data channel for the transmission of data packets from a primary station to a secondary station, a plurality of downlink control channels are used to signal information relating to packet transmission. A problem with this scheme is that with a fixed control channel allocation system throughput (T) for a given offered load (O) can be significantly reduced in a worst case scenario (for example two secondary stations using the same control channel). One solution uses an indicator signal to inform a secondary station of the control channel it should receive, but this adds significant complexity to the system. The present invention provides a simpler scheme having similar benefits by shuffling the allocation of control channels to secondary stations, according to a defined sequence for each secondary station, thereby avoiding the worst case scenario.

TECHNICAL FIELD

The present invention relates to a radio communication system andfurther relates to primary and secondary stations for use in such asystem and to a method of operating such a system. While the presentspecification describes a system with particular reference to theUniversal Mobile Telecommunication System (UMTS), it is to be understoodthat such techniques are equally applicable to use in other mobile radiosystems.

BACKGROUND ART

There is a growing demand in the mobile communication area for a systemhaving the ability to download large blocks of data to a Mobile Station(MS) on demand at a reasonable rate. Such data could for example be webpages from the Internet, possibly including video clips or similar.Typically a particular MS will only require such data intermittently, sofixed bandwidth dedicated links are not appropriate. To meet thisrequirement in UMTS, a High-Speed Downlink Packet Access (HSDPA) schemeis being developed which may facilitate transfer of packet data to amobile station at up to 4 Mbps.

A particular problem with the design of the HSDPA scheme is themechanism for informing a MS of the presence of a data packet for it toreceive and providing information relating to the packet (typicallyincluding details of the particular transmission scheme employed, forexample spreading code, modulation scheme and coding scheme). Ascurrently proposed, this information is signalled on one of fouravailable downlink control channels, distinguished by their spreadingcodes. The MS is instructed to decode one of the control channels by atwo-bit indicator signal which is transmitted on a low data ratededicated downlink channel (the signal being inserted by puncturing).The MS then monitors the same control channel for subsequent packets ina burst.

This scheme conveniently supports the scheduling of up to four packetsto different MSs in the same time interval. Use of the indicator signalis intended to reduce the complexity of the MS and its powerconsumption, as the MS only needs to monitor the dedicated downlinkchannel for the indicator signal instead of having to receivecontinuously all four control channels. However, there are significantdrawbacks with the use of the indicator signal. One drawback is that anadditional slot format is required for the dedicated downlink channel(to accommodate the extra signal), which adds complexity. Anotherdrawback is that the transmission power required for the indicatorsignal can be relatively high to ensure reliable reception of the signaleven at the edge of a cell.

One solution which avoids the use of an indicator signal is for each MSto be allocated one of the four control channels, which it thencontinuously monitors. However, if more than one MS is allocated thesame control channel the flexibility of packet scheduling is restricted.Another solution is the provision of one control channel for each MS;however, the potentially large number of channels required could use upexcessive system resources.

DISCLOSURE OF INVENTION

An object of the present invention is to provide an improved arrangementwhich does not require an indicator signal or provision of a largenumber of control channels.

According to a first aspect of the present invention there is provided aradio communication system having a data channel for the transmission ofdata packets from a primary station to a secondary station and aplurality of control channels for signalling of control informationrelating to the data packets from the primary station to the secondarystation, wherein the primary station comprises means for allocating oneof the control channels to the secondary station and means for changingthe allocated control channel according to a defined sequence, and thesecondary station comprises means for monitoring the currently allocatedcontrol channel to determine information about packet transmissions.

By changing the control channel allocation, system performance isgreatly enhanced under worst-case conditions without the need for anindicator signal, which introduces significant extra complexity. Thedefined sequence may repeat regularly, for example once per frame, andmay use as a timing reference a common downlink channel, for example asynchronisation channel in UMTS.

When control channels are allocated to a plurality of secondarystations, their respective defined sequences are preferably alldifferent (provided the number of secondary stations is not too great),and some (but not necessarily all) of the sequences may include only asingle control channel.

According to a second aspect of the present invention there is provideda primary station for use in a radio communication system having a datachannel for the transmission of data packets from the primary station toa secondary station and a plurality of control channels for signallingof control information relating to the data packets from the primarystation to the secondary station, wherein means are provided forallocating one of the control channels to the secondary station and forchanging the allocated control channel according to a defined sequence.

According to a third aspect of the present invention there is provided asecondary station for use in a radio communication system having a datachannel for the transmission of data packets from a primary station tothe secondary station and a plurality of control channels for signallingof control information relating to the data packets from the primarystation to the secondary station, wherein means are provided fordetermining which of the control channels is allocated to the secondarystation, the allocated control channel being changed according to adefined sequence, and for monitoring the currently allocated controlchannel to determine information about packet transmissions.

According to a fourth aspect of the present invention there is provideda method of operating a radio communication system having a data channelfor the transmission of data packets from a primary station to asecondary station and a plurality of control channels for signalling ofcontrol information relating to the data packets from the primarystation to the secondary station, the method comprising the primarystation allocating one of the control channels to the secondary stationand changing the allocated control channel according to a definedsequence, and the secondary station monitoring the currently allocatedcontrol channel to determine information about packet transmissions.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a block schematic diagram of a radio communication system; and

FIG. 2 is a graph of worst-case system throughput T in millions of bitsper second (Mbps) against offered load O in Mbps for various controlchannel schemes.

MODES FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, a radio communication system comprises a primarystation (BS) 100 and a plurality of secondary stations (MS) 110. The BS100 comprises a microcontroller (μC) 102, transceiver means (Tx/Rx) 104connected to antenna means 106, power control means (PC) 107 foraltering the transmitted power level, and connection means 108 forconnection to the PSTN or other suitable network. Each MS 110 comprisesa microcontroller (μC) 112, transceiver means (Tx/Rx) 114 connected toantenna means 116, and power control means (PC) 118 for altering thetransmitted power level. Communication from BS 100 to MS 110 takes placeon a downlink channel 122, while communication from MS 110 to BS 100takes place on an uplink channel 124.

The general characteristics of the UMTS HSDPA were outlined above andare summarised here for clarity:

-   -   There are (low data rate) dedicated uplink 124 and downlink 122        channels between a BS 100 and each MS 110 in its cell.    -   A specific downlink channel 122 is used for high-speed        transmission of data packets. This channel is subdivided into        Transmission Time Intervals (TTIs), where each TTI is the time        taken for transmission of a data packet. In UMTS the duration of        a TTI is 2 ms, and this time period is also identified as a        sub-frame (there being three time slots in a sub-frame, and        hence 15 time slots in a 10 ms frame).    -   Up to four downlink control channels are provided, distinguished        by their spreading codes and each relating to transmission        parameters of a data packet. Hence, up to four data packets can        be transmitted simultaneously per TTI.        -   The requirement for being able to schedule four data packets            to different stations 110 in the same TTI is to allow high            system throughput to be achieved in a cell in which some            stations 110 do not have the ability to receive all of the            HSDPA downlink resource. For example, some stations 110 may            be able to receive only 5 spreading codes when there are up            to 15 available.    -   A mechanism is provided for indicating to a particular MS 110        that it is scheduled to receive a data packet and for indicating        which control channel it should listen to in order to determine        how to receive the packet.

As described above, one possible mechanism is the transmission of anindicator signal on the dedicated downlink channel 122 to inform a MS110 of the transmission of a data packet. However, this mechanism has anumber of problems.

As an alternative each MS 110 could be allocated one of the controlchannels to monitor, thereby avoiding the need for an indicator signal.However, if more than one MS 110 is allocated to a particular controlchannel the flexibility of packet scheduling is restricted. For example,consider two mobile stations 110, each with data to be sent but bothallocated the same control channel. It would generally be desirable tosend data simultaneously to both stations 110. However as both stationsare sharing a control channel, only one packet can be sent at a time.Given that packet transmission is often bursty in nature, this situationis likely to continue for several TTIs and the system throughput couldbe only 50% of the maximum. Greater scheduling flexibility could beintroduced by requiring each MS 110 to monitor two control channels, butat the cost of increased MS power consumption.

In a system made in accordance with the present invention, this problemis addressed by shuffling the allocation of control channels from oneTTI to the next. Hence, if two stations 110 share a control channel inone TTI they will have different ones in the next TTI. If such a schemeis applied to the example above of two active stations 110, then awell-designed shuffling scheme should be able to reduce the probabilityof an “allocation collision” to 1/N_(con), where N_(con) is the totalnumber of control channels (four in the above examples). The maximumloss in throughput would then be 0.5/N_(con), or 12.5% with N_(con)=4(compared to 50% without shuffling).

Some examples of how shuffling may be done will now be presented,although the schemes themselves are not necessarily optimal.

First consider the case of two control channels and four stations 110.The allocation of control channels to each station (0 to 3) for each TTI(0 to 4) in a 10 ms frame is: TTI station 0 1 2 3 4 0 0 0 0 0 0 1 1 1 11 1 2 0 1 0 1 0 3 1 0 1 0 1This scheme could either repeat in the next frame or be made into alonger cycle.

Next, consider an extension of the above scheme to the case of twocontrol channels and six stations 110: TTI station 0 1 2 3 4 0 0 0 0 0 01 1 1 1 1 1 2 0 1 0 1 0 3 1 0 1 0 1 4 0 0 1 1 0 5 1 1 0 0 1

As a third example, consider a system with four control channels andtwelve stations 110: TTI station 0 1 2 3 4 0 0 0 0 0 0 1 1 1 1 1 1 2 2 22 2 2 3 3 3 3 3 3 4 0 1 2 3 0 5 1 2 3 0 1 6 2 3 0 1 2 7 3 0 1 2 3 8 0 32 1 0 9 1 0 3 2 1 10 2 1 0 3 2 11 3 2 1 0 3It may not be required to have a unique shuffling pattern for each MS110. In this case it seems preferable to take the station number as ashuffling pattern identifier, and to assign stations 110 to each patternin ascending sequence. Hence, for small numbers of stations 110 all (ormost) of them will have a constant control channel allocation. Althoughconvenient, it is clearly not a requirement of the present inventionthat any of the shuffling patterns relate to a constant control channelallocation.

The shuffling pattern of the third example can be represented asn _(CCH)=[(a×n _(TTI))+b] mod N _(CCH)where: n_(CCH) is the number of the control channel to be used; N_(CCH)is the total number of control channels available; n_(TTI) is the numberof the TTI in the frame; a is a parameter taking values 0, 1 or 3; and bis a parameter taking values 0, 1, 2 or 3.

Simulations of worst-case system throughput were performed using theshuffling pattern of the third example. The following are the mainassumptions made for the detailed specification of the simulated system:

-   -   Hexagonal 19-cell layout, with a representative segment of the        central cell considered for the throughput estimate.    -   Number of stations 110 (per cell)=12    -   Static TTI=3 slots (2 ms)    -   Propagation exponent=3.76    -   Single path Rayleigh fast fading model (flat spectrum)    -   Channel conditions stationary during a TTI    -   MS speed 3 km/h    -   Standard deviation of log-normal shadowing=8 dB    -   Shadowing correlation between sites=0.5    -   30% of BS power allocated to common channels etc in all cells    -   70% of BS power allocated to HSDPA in all interfering cells    -   70% of BS power available to HSDPA in wanted cell    -   Overheads due to dedicated channels associated with HSDPA not        considered    -   10 spreading codes available for HSDPA    -   MS capability: 5 spreading codes    -   Spreading factor=16    -   Available Modulation and Coding Schemes (MCS):        -   1. QPSK 1/4 rate        -   2. QPSK 1/2 rate        -   3. QPSK 3/4 rate        -   4. 16-QAM 1/2 rate        -   5. 16-QAM 3/4 rate    -   Equal transmission power per code    -   Frame error rate computed from Signal to Interference Ratio        (SIR) and block code performance bounds.

To represent streaming services it is assumed that the offered load iscomprised of one constant rate data stream per MS 110. For simplicityequal bit rates are also assumed for each data stream. The data for eachuser is assumed to arrive at a queue in the BS 100, and the queue isupdated every TTI. It is assumed that one CRC (Cyclic Redundancy Check)is attached per packet.

As a default, Chase combining of re-transmissions is assumed. Anerroneous packet is re-transmitted with the same MCS. Perfect maximumratio combining is assumed, and the final SIR is computed as the sum ofthe SIRs of the two packets to be combined.

The simulated scheduler is novel, and is intended to maximise systemthroughputs. This is done by giving priority to the users which can sendthe largest packets. For the case of a fixed transmission time this isequivalent to scheduling according to the maximum bit rate that can beprovided to each user. The packet size which can be sent is determinedmainly by the CIR (Carrier to Interference Ratio). This determines theprobability of successful transmission which will be obtained with anygiven modulation and coding scheme. For each possible scheme aneffective packet size can be calculated asP_(size)=N_(code)×P_(bits)(1−BLER), where N_(code) is the number ofchannelisation codes which can be used, P_(bits) is the number of bitstransmitted per channelisation code, and BLER is the estimated blockerror rate for the given transmission scheme. N_(code) is most likely tobe determined by the capability of of the MS 110 to receive a givennumber of channelisation codes simultaneously, but it could be limitedby the number of codes allocated by the system. There will also be anupper bound on N_(code)×P_(bits) due to the amount of data in the queuewaiting to be sent to that MS.

A viable approach is to calculate the maximum value of P_(size) for eachMS 110 at each TTI (sub-frame). Then sort this into a list in order ofdecreasing P_(size), then schedule transmission of packets to each MSstarting at the front of the list and working down it until all theavailable downlink resource is assigned. Further variations are possiblein which the power assigned to each packet might be adjusted to optimiseperformance.

Such a scheduler has the aim of maximising total throughput for thosestations 110 which have been granted access to HSDPA.

Other general assumptions are that:

-   -   A data packet for any user can be allocated to any        channelisation code.    -   More than one channelisation code can be allocated to one user.    -   The code block size is equal to the amount of data that can be        sent with one channelisation code, which means that a “packet”        may comprise multiple code blocks sent in parallel within one        TTI.    -   Re-transmissions and first transmissions to the same user are        not allowed within the same TTI.    -   The modulation, coding scheme and power level for first        transmissions are chosen to maximise throughput.    -   All re-transmissions are scheduled before first transmissions,        thus giving them a higher priority, and no first transmissions        are allowed to a MS 110 while any re-transmissions remain to be        sent.    -   The modulation and coding scheme of a re-transmission is the        same as for the first transmission.

The results of the simulation are shown in FIG. 2, as a graph of systemthroughput T in millions of bits per second (Mbps) against offered loadO in Mbps. Results are shown for three control channel schemes. In thefirst, shown as a solid line, each MS 110 is allocated a single controlchannel (and all stations 110 are allocated the same control channel forthis worst-case scenario). In the second, shown as a dashed line, anindicator signal is used to inform a MS 110 of which control channel tomonitor, hence each MS 110 is effectively monitoring all four channels.In the third, shown as a chain-dashed line, a shuffling control channelallocation is used as shown for the third example above.

The results clearly show that the first scheme can result insignificantly degraded performance under worst-case conditions, whilethe second and third schemes have comparable performance. Although theuse of an indicator signal provides the best results, the results fromuse of a shuffling control allocation scheme are not significantly worsewhile, as discussed above, providing significant simplifications tosystem implementation.

In embodiments of the present invention, a range of modifications to theschemes described above are possible. The BS 100 could agree a shufflingpattern with each MS 110. Then if a MS 110 correctly decodes the controlchannel for the current TTI, the principle (currently employed in HSDPA)that it decodes the same control channel in the next TTI should beinterpreted to mean that in the next TTI the MS 110 decodes the controlchannel indicated by its assigned shuffling pattern (which may or maynot be the same as that for the current TTI).

The time duration of the control channel allocation cycle need not beone frame but could be any convenient length. The timing reference forthe shuffling sequence could be a common downlink channel such as asynchronisation channel.

The protocol could be modified so that if a MS 110 detects a controlchannel transmission but the CRC fails, the MS 110 sends a NACK(negative acknowledgement), which could be different from that sent whenthe CRC for a data packet fails. This would reduce the powerrequirements for control channel transmission, since a higher error ratecould then be tolerated. This would give the BS 100 some flexibility inchoosing the control channel power, but it might restrict the use ofnon-self-decodable redundancy versions for the re-transmission of datapackets (where the original data cannot be deduced from there-transmission alone).

The transmission of control channels could be restricted in time to oneout of every N TTIs (at least for the first packet of a group). Thiswould allow the MS 110 to save some power by not continuously decoding acontrol channel. The first allocated TTI could be a MS-specificparameter. The restriction could be relaxed when data transmissionstarts (e.g. when the BS 100 has received an ACK (acknowledgement) forthe first packet in a sequence of packets). This event could set atimer. When the timer expires the situation could revert to use of everyN^(th) TTI. A range of sequences other than one in every N TTIs couldalso be used.

There is an alternative method for resolving the scheduling problemwhere more than one MS 110 needs to be sent data at the same time, butthey have been allocated the same control channel. The format of thecontrol channel is modified to contain an indication that a differentphysical layer message is intended for the MS 110. In a UMTS embodimentthis is preferably as an alternative (rather than an addition) to theinformation on the format of a data packet to be sent on the downlinkdata channel. This indication could a single bit flag. The physicallayer message in this case would be an instruction to change one or moreof the control channel(s) which the MS 110 should monitor, from a amonga pre-defined set. In a UMTS embodiment the existing ACK/NACK signalling(currently intended to relate to data on the downlink data channel)could be used to indicate whether the physical layer message wasreceived correctly by the MS 110. Alternatively different codewordscould be used in the ACK/NACK field for this purpose. Some of theexisting control channel structure could be used (for example datafields identifying the intended recipient, or CRCs for error detection).Other physical layer messages might be conveyed in the same way, as analternative to using the control channel to describe the format of apacket on the data channel. This may require a multi-bit indication/flagof message type. Preferably substantially the same format would be usedon control channel, irrespective of the message contents.

A further alternative is to add a data field to the control channel sothat a message to change the control channel allocation can be sent atthe same time as a data packet. This avoids loss of data transmissioncapacity in the downlink. Such a message could indicate that, startingwith a future TTI, the control channel should change. In this case itwould be desirable to limit the size of the message needed (e.g. to oneor two bits). Therefore the change could be to a new channel from asmall set of available channels or to a new channel which is the nextone in a defined sequence.

The description above related to the BS 100 performing a variety ofroles relating to the present invention. In practice these tasks may bethe responsibility of a variety of parts of the fixed infrastructure,for example in a “Node B”, which is the part of the fixed infrastructuredirectly interfacing with a MS 110, or at a higher level in the RadioNetwork Controller (RNC). In this specification, the use of the term“base station” or “primary station” is therefore to be understood toinclude the parts of the network fixed infrastructure involved in anembodiment of the present invention.

As well as its application in a FDD (Frequency Division Duplex) systemas described above, the present invention could be applied in othertypes of communication system. For example, it could be used in a TimeDivision Duplex (TDD) system with the modification that the physicalchannels used may also be distinguished by their use of different timeslots or other defined time interval.

1. A radio communication system having a data channel for thetransmission of data packets from a primary station to a secondarystation and a plurality of control channels for signalling of controlinformation relating to the data packets from the primary station to thesecondary station, wherein the primary station comprises means forallocating one of the control channels to the secondary station andmeans for changing the allocated control channel according to a definedsequence, and the secondary station comprises means for monitoring thecurrently allocated control channel to determine information aboutpacket transmissions.
 2. A system as claimed in claim 1, characterisedin that means are provided for regularly repeating the defined sequence.3. A system as claimed in claim 2, characterised in that the radiochannels are divided into time frames and in that means are provided forrepeating the defined sequence once per frame.
 4. A system as claimed inany one of claims 1 to 3, characterised in that a timing reference forrepetition of the defined sequence is provided by a common downlinkchannel.
 5. A primary station for use in a radio communication systemhaving a data channel for the transmission of data packets from theprimary station to a secondary station and a plurality of controlchannels for signalling of control information relating to the datapackets from the primary station to the secondary station, wherein meansare provided for allocating one of the control channels to the secondarystation and for changing the allocated control channel according to adefined sequence.
 6. A primary station as claimed in claim 5,characterised in that means are provided for scheduling data packets fora plurality of secondary stations by giving priority to the largest datapackets.
 7. A primary station as claimed in claim 5 or 6, characterisedin that means are provided for allocating control channels for aplurality of secondary stations according to a plurality of respectivedefined sequences, all of which are different.
 8. A primary station asclaimed in claim 7, characterised in that not all of the definedsequences include more than one control channel.
 9. A primary station asclaimed in any one of claims 5 to 9, characterised in that means areprovided for transmitting at least one of the control channels for onlya proportion of the time that data packets are transmitted.
 10. Asecondary station for use in a radio communication system having a datachannel for the transmission of data packets from a primary station tothe secondary station and a plurality of control channels for signallingof control information relating to the data packets from the primarystation to the secondary station, wherein means are provided fordetermining which of the control channels is allocated to the secondarystation, the allocated control channel being changed according to adefined sequence, and for monitoring the currently allocated controlchannel to determine information about packet transmissions.
 11. Asecondary station as claimed in claim 10, characterised in that meansare provided for transmitting a negative acknowledgement to the primarystation to indicate that the allocated control channel is successfullydetected but cannot be correctly received.
 12. A secondary station asclaimed in claim 11, characterised in that the negative acknowledgementis a different signal from that used to indicate that a data packetcould not be correctly received.
 13. A method of operating a radiocommunication system having a data channel for the transmission of datapackets from a primary station to a secondary station and a plurality ofcontrol channels for signalling of control information relating to thedata packets from the primary station to the secondary station, themethod comprising the primary station allocating one of the controlchannels to the secondary station and changing the allocated controlchannel according to a defined sequence, and the secondary stationmonitoring the currently allocated control channel to determineinformation about packet transmissions.